Lancer (ランサー,Ransā?), Knight of the Lance and Heroic Spirit of the Lance, is one of the Three Knight classes among the seven standard Servant classes summoned for the Holy Grail War. Servants placed within this class, many of which are Heroic Spirits who were knights, are very agile, display excellent close combat skills, show proficiency in hit-and-run tactics that capitalize on range and speed, and are skilled with long-range melee weapons such as spears and lances. The qualifying conditions to be placed within the class are second only to the Saber class, requiring overall excellent attributes and a high agility attribute. Their only particular class ability is Magic Resistance, and although they "tend to be less flashy than the other classes", they display great reliability.[1][2]

Class
A "Servant Class" is a container prepared for materialising a Heroic Spirit that is summoned into the current era as a Servant.
There are seven Servant Classes: Sabre, Lancer, Archer, Rider, Caster, Assassin and Berserker. Since there cannot be repeats of any one class, the seven Servants participating in a Holy Grail War will be of different classes.
Heroic Spirits determine their classes on the basis of their characteristics, and then summoned as the Servant of the corresponding class. For instance, the only class appropriate for Artoria is Sabre, but Hercules, who was said in legends to be proficient with all weapons, can be summoned into all classes but Caster. The affinity between the Master and the Heroic Spirit will determine which class the Heroic Spirit will be summoned into. However, the Einzberns, as one of the "Three Founding Families" and well-informed of the rules of the Servant system, was able to preemptively interfere with the system and summon Hercules as Berserker.
Among all the classes, Sabre, Lancer and Archer are also called the "Three Knights". They are frequently Heroic Spirits from the Age of Gods when sorceries were common. Due to high magic resistance, modern magi are unable to inflict damage to them.
The Assassin class is special, as the only qualifying Heroic Spirits are the Hassan al-Sabbah's.

Saber
Knight of the Sword. The compatible Heroic Spirit naturally needs to possess legends as a knight of sword, as well as the highest levels of attributes in all but MGI. Class Abilities include Magic Resistance and Riding. Also, compatible Heroic Spirits usually excel in delivering powerful burst attacks.

Lancer
Knight of the Lance. Its high qualifying conditions are second only to Sabre. Attributes are excellent overall, in addition, AGI must be high. Of course, Servants of this class are proficient in hit-and-run tactics that capitalise on range and speed. This class includes many Heroic Spirits who were knights.

Archer
Knight of the Bow. A class that excels in possessing powerful Noble Phantasms. The qualifying condition is not attributes, but the possession of powerful projectile weapons or special abilities related to projectile weapons. Highly effective as scouts. They have not only the Magic Resistance ability of the Knight Classes, but also the class ability of Independent Action.

Rider
Mounted soldier. Heroic Spirits who rode mounts (not limited to living creatures) qualify for this class. Although attributes tend to be lower compared to the Three Knight Classes, this is compensated by the abilities of the mount described in legend. Besides Magic Resistance, class abilities also include high Riding.

Caster
Magus. Qualifying conditions only include mastery of sorceries of the highest calibre. Due to this trait, the qualifying Heroic Spirits tend to have low combat abilities. In addition, since the majority of Servants have Magic Resistance, this class is thought to be the weakest of all.

Assassin
Assassin. The only qualifying Heroic Spirits are the generations of Hassan al-Sabbah's, and one of them would be summoned. All of them do not have glorious legends as heroes, thus possess low attributes. Class ability is Presence Concealment, and the use of this ability during battle is the lifeline of Assassin.

Berserker
Mad Warrior. Heroes who have gone berserker during battle qualify for this class. Normally it is considered ideal if a Servant can wield power at the level of the original Heroic Spirit, but "Mad Enhancement" allows for the attribute enhancement of the Servant beyond the ability of the original at the cost of sanity. A class originally used to enhance weak Heroic Spirits.

Lancer [Servant]
Heroic Spirit of the lance. Requires supreme agility, as well as excellent close combat skills. Though they tend to be less flashy than the other classes, they are a very reliable type of Servant.
The Lancer in the story has a rough and tumble personality, but at his core he believes in justice and is easy to get along with. He is an agreeable youth acting out the role of one gentle in disposition yet strong in body.
His true identity is the great Celtic hero, Cu Chulainn.
If the stage of this Holy Grail War were anywhere but Japan, he would be a shining existence on par with Heracles and King Arthur.
As was mentioned in the story, he is a Heroic Spirit that specializes in survival, capable of returning alive no matter how hopeless the situation. Unfortunately, this time luck wasn't on his side.
It is said that in the midst of battle he would unleash a fearsome war cry, and at the same time his body would swell and bulge with muscle as his behavior became like that of a demon.
However, though he possesses the qualifications to be a Berserker there is no doubt that the Lancer class suits him best.
He responded to the summoning solely because he wanted to engage in a valiant fight to the death, but this simple wish was never granted to him.
He is a tragic Servant, who embodies the cynical notion that "wishes don't come true".